The present invention relates to a coaxial starter including an armature shaft of an electric motor, an output shaft with a pinion fixed to the end thereof, and an operating rod of an electromagnetic switch which are arranged coaxially with one another and, in particular, to improved coupling means for coupling a planetary gear frame to a clutch outer.
FIG. 2 shows a section view of a coaxial starter according to the prior art. In FIG. 2, DC electric motor 1 is constructed in the following manner: a permanent magnet 2 is mounted to a yoke 3. An armature 4 is composed of an armature iron core 5 fixed to a hollow armature shaft 6, an armature coil 7, and a commutator 8. A rear bracket 9 is mounted to the yoke 3. The armature shaft 6 is supported through bearings 10 and 11 by the front end support portion 3a of the yoke 3 and also by the rear bracket 9. A brush holder 12 presses a brush 13 against the commutator 8.
A planetary gear reduction device 15 is used to reduce the rotational speed of the armature shaft 6 and then transmit the reduced speed. The reduction device 15 is constructed in the following manner: A sun gear 6a is composed of a pinion which is formed in the end portion of the armature shaft 6. A frame member 16 is mounted to the yoke 3 and having an internal gear 17 for serving as an inner bracket as well. A plurality of planetary gears 18 mesh with the sun gear 6a and revolve on its axis as well as mesh with the internal gear 17 and revolve round the internal gear 17 in a reduced manner, and a planetary gear frame 19 is supported through a ball bearing 33 by the armature shaft 6 and in turn supports the planetary gears 18 through a bearing 21 by a fixed support pin 20 to transmit the revolution of the planetary gears 18 round the internal gear 17.
An output shaft 22, one end of which is supported through a bearing 23 in a hollow bore formed in the armature shaft 6. And a pinion 24 is spline coupled to the other end of the output shaft 22 and is covered by a stopper 25 so that the pinion 24 is prevented from slipping off of the output shaft 22.
An over-running clutch 26 is used to transmit the output rotation of the planetary gear reduction device 15 to the output shaft 22 by means of unidirectional rotation. The over-running clutch 26 is constructed in the following manner: The inside diametrical portion of one end of the clutch outer 27 is shrinkage fitted on to the outer periphery of a projection provided on and from one end of the planetary gear frame 19. And, the interferences of the respective fitted portions 28 of the clutch outer 27 and planetary gear frame 19 are set such that they are allowed to slide on each other if a rotational torque exceeds a predetermined value. A clutch inner 29 has an inner peripheral helical spline 29a in mesh with a helical spline 22a formed in the output shaft 22. The outer end of the clutch inner 29 is supported through a bearing 34 by the frame member 16. A plurality of rollers 30 are interposed between the clutch outer 27 and clutch inner 29. A coupling cap 31 presses against a bisected stiffening plate 32 and is staked to the outer periphery of the clutch outer 27.
The front portion of the output shaft 22 is supported through a bearing 35 by the clutch inner 29 in such a manner that it is movable in the axial direction thereof. A return spring 36 is used to move back or return the output shaft 22 after it has been advanced, and a front bracket 37 is mounted to the frame member 16 by a bolt 38.
An electromagnetic switch 40 is coaxially mounted to the rear end of the DC electric motor 1 and is constructed in the following manner: A case 41 surrounds electromagnetic switch 40 and forms a magnetic circuit and is mounted to the rear bracket 9. A fixed iron core 42 is fixed within the front end portion of the case 41, and an excitation coil 43 is wound around a bobbin supported by the case 41 and fixed iron core 42. A plunger 44 forms a movable iron core and is supported at the end of an operating rod 45 formed of a non-magnetic tubular rod. A movable contact supported by the operating rod 45 via an insulating material 47 and is pressed in the advancing direction thereof by a compression spring 48.
A push rod 49 is pressed by a compression spring 50. The rear end thereof is inserted into the operating rod 45 and the front end thereof is confronted by a steel ball 51 disposed in a recess at the real end portion of the output shaft 22. A compression spring which pushes and holds the steel ball 51 against the recess. A pair of fixed contacts 53 are embedded into and held by an insulating material 54 fixed to the rear bracket 9 and are opposed to the movable contact 46. A terminal bolt portion 55 is formed integrally in one of the fixed contacts 53 and is exposed externally, and an end cover.
The end cover 56, case 41, rear bracket 9, yoke 3 and frame member 16 are coupled/assembled by a through bolt 57.
Next, description will be given of the operation of the conventional coaxial starter constructed in the above-mentioned manner. If a starting switch provided in a vehicle is closed, then the excitation coil 43 is energized to thereby advance or move the plunger forwardly. As a result of this, the operating rod 45 is advanced together with the plunger 44, the push rod 49 is advanced through the compression spring 50, and the output shaft 22 is also moved forwardly through the steel ball 51. Then, the pinion 24 that has been advanced together is brought into mesh with a ring gear provided in an engine. At that time, the movable contact 46 that has been advanced is pressed against the two fixed contacts 53 to thereby energize the armature coil 7 of the DC electric motor 1. In this manner, the armature 4 is started to rotate, the rotation of the armature shaft 6 is reduced by the planetary gear reduction device 15, the reduced rotation is transmitted to the clutch outer 27 of the over-running clutch 26, and the rotation is transmitted to the output shaft 22 from the clutch inner 29. This starts the rotation of the engine through the pinion 24.
When the engine is started and the electrical energization to the excitation coil 43 is cut off, then the output shaft 22 is moved back or returned by the return spring 36 to thereby disconnect the pinion 24 from the ring gear, and at the same time the movable contact 46 is moved back through the push rod 49, compression spring 50 and working rod 45 to part away from the fixed contacts 53.
The above-mentioned planetary gear frame 19 and clutch outer 27 are fitted to each other in such a manner that they are allowed to slide over each other if a rotational torque exceeds a predetermined value. If an excessive shock stress occurs in a start drive system ranging from the DC electric motor 1 to the engine, for example, if the engine is started during coasting rotation, then the planetary gear frame 19 and clutch outer 27 are allowed to slide over each other in a fitted portion 28 thereof to thereby absorb the shock stress so as to prevent damage or breakage of the pinion, ring gear and the like.
As described above, according to the shock absorbing mechanism used to deal with an abnormal load applied in the above-mentioned conventional coaxial starter, the planetary gear frame 19 and clutch outer 27 are arranged so as to be slidable on the shrinkage fitted surface thereof. However, if a slight shift is produced in an axial direction during such sliding on the shrinkage fitted surface and such shift is repeatedly produced, for example, a gap between the end of the support pin 20 and the front end support portion 3a of the yoke 3 is then caused to disappear to thereby impede the rotational action of the planetary gear frame 19, that is, to worsen the performance of the mechanism. In the worst case, the armature 4 is prevented from rotation.